1. Field of the Invention
The invention is related to the field of communication networks and, in particular, to negotiating for control over a Push-to-Talk (PTT) call between an Open Mobile Alliance (OMA) Push-to-Talk over cellular (PoC) network and a Project (P25) network.
2. Statement of the Problem
Push-to-Talk (PTT) over Cellular (PoC) is a walkie-talkie type service that is provided over a cellular phone network. PTT calls are half duplex communications meaning that voice communications are transmitted one way from a speaking party to one or more listening parties, as opposed to a two-way transmission between the speaking party and the listening parties. To place a PTT call, the speaking party pushes a button on his/her mobile device, and the mobile device transmits a PTT call request to the PoC network. The PoC network grants the speaking party the floor for the call, and the mobile device is permitted to transmit voice communications to the listening parties. While the speaking party has the floor, the PoC network does not permit the listening parties to transmit voice communications to the speaking party because of the half duplex connection. After the speaking party is done talking, a listening party may then obtain the floor from the PoC network to respond to the speaking party.
The Open Mobile Alliance (OMA) is a standards body that defines standards for PoC networks (other proprietary standards also exist). The OMA PoC standards set forth rules for assigning control over initial transmission for PTT calls within the PoC network. A PoC network comprises one or more PTT servers that serve PTT calls. The OMA PoC standards define which PTT server is granted control, and the controlling PTT server determines which party is granted the floor for a PTT call. The controlling PTT server forwards voice communications from the party having the floor, and discards any voice communications from other parties to the PTT call that do not have the floor.
Project 25 (P25) is a suite of standards for digital radio communications for use by federal, state/province, and local public safety agencies in North America to enable them to communicate with other agencies and mutual aid response teams in emergencies. P25 was established to address the need for common digital public safety radio communications standards for first responders and Homeland Security/Emergency Response professionals. The P25 suite of standards involves digital Land Mobile Radio (LMR) services commonly used by police, first responders, and other public safety organizations.
It may be desirable for P25-compliant LMR devices to communicate with mobile devices of an OMA PoC network for PTT calls. The P25 suite includes the Inter-Subsystem Interface (ISSI) which defines the interface between Radio Frequency (RF) subsystems and wide area networks, such as an OMA PoC network. Like the OMA PoC standards, the ISSI standards set forth rules for assigning control over initial transmission for PTT calls within the P25 network. The PTT server in a P25 network is referred to as the Radio Frequency Subsystem (RFSS). The ISSI standards define which RFSS is granted control, and the controlling RFSS determines which party is granted the floor for a PTT call.
Problems would be encountered when a one-to-one PTT call is placed from an OMA PoC network to a P25 network. According to the OMA PoC standards, the PTT server that is serving the calling party for a PTT call is assigned as the controlling PTT server. According to P25 ISSI standards, the RFSS that is serving the called party for a PTT call is assigned as the controlling PTT server. Thus, when a call is established from a calling party in an OMA PoC network to a called party in a P25 network, both the PTT server in the OMA PoC network and the RFSS in the P25 network assume the role of controlling server. The PTT server in the OMA PoC network grants the calling party the floor for initial transmission for the PTT call, while at the same time the RFSS in the P25 network grants the called party the floor for initial transmission. When a voice communication from the called party reaches the PTT server in the OMA PoC network, the PTT server discards the communication because the called party doesn't have the floor in the OMA PoC network. Likewise, when a voice communication from the calling party reaches the RFSS in the P25 network, the RFSS discards the communication because the calling party doesn't have the floor in the P25 network. Due to the conflict between the OMA PoC and P25 ISSI standards, no voice communications are exchanged between the parties resulting in a communication breakdown.